In many shops, boutiques, shopping centers, and rental shops and libraries storing recorded video tapes, compact discs and records, shoplifting detecting systems have been installed so as to prevent shoplifting or other improper taking-out of merchandise, or software and books to be lent out. The shoplifting detecting systems are roughly classified into those of radio-wave type, and those of magnetic type. As a magnetic-type device for preventing shoplifting or other improper taking-out of books, for example, a strip-like magnetic sensor sheet S including a coder that has been demagnetized is attached to a book B as shown in FIG. 14. Upon purchase or renting, the book B is placed on a writer D so that a magnetic effect is given to the coder, thereby to suppress the function of a transponder of the magnetic sensor sheet S. If the customer properly finishes processing of transaction for the book Ba at a checkout resister or lending section where the magnetic effect is given to the coder of the magnetic sensor sheet S, an alarm system Aa does not go off when the customer carrying the book Ba passes through a detection gate G installed at the entrance of the shop. If the customer improperly takes out the book Bb without going through the checkout register or lending section, the detection gate G senses the transponder of the magnetic sensor sheet S when the customer passes through the gate G, and the alarm system Ab emits light or sound to thus generate an alarm.
Although the size of the magnetic sensor sheet S as a known shoplifting prevention device has been reduced, customers can still easily recognize the presence of the magnetic sensor sheet S, and there is a possibility that the sensor sheet S is easily removed and thrown away on purpose. Further, the cost of processing and attaching the shoplifting prevention device tends to be high since separate components or markers used for preventing shoplifting must be attached to individual items of merchandise. Also, the appearance of the goods is likely to deteriorate due to the presence of such devices.